As a technique of removing extraneous salts resulted from the reaction of silver salt with a halide salt in a process of preparing a silver halide photographic emulsion (hereinafter referred to as emulsion), a so-called desalting method, there has been employed a coagulation process in which a protective colloid is coagulated with a coagulating agent and the resulting supernant solution is removed, as disclosed in JP-A 48-13057 (the term "JP-A" means an "unexamined published Japanese patent application), JP-A 59-166939, 60-220337, 60-52844, 62-212642, 62-237993, 63-214743 and 2-272442.
In this method, however, a coagulating agent is adhered to the grain surface to affect adversely spectral sensitization and chemical sensitization. As a result of adhesion of a coagulatig agent to grains, it becomes impossible to cause the grains to grow as seed grains and even if grains have been grown up, it become difficult to form silver halide having an intended internal structure, since an interface is formed by the coagulating agent inside the grain.
In order to avoid these unfavorable effects, there has been employed a method of desalting without the use of a additive such as a coagulating agent. A noodle washing method has been long-employed so far, in which an emulsion is cooled down to be set and shredded in the form of noodle to be washed is disclosed, for example, in JP-A 47-44487 49-46255. In this method, however, it needs a longer time to complete the desalting and moreover, an incorporation of a large amount of water into the emulsion resulted in an increase in volume thereof, causing to lower efficiency in the subsequent emulsion-making process such as chemical ripening, storage and transportation thereof.
There was also reported a ultrafiltration by use of a semipermeable membrane, as disclosed in JP-A 57-209823, 59-43727, 62-113137, 2-172816, 2-172817 and 3-140946. This technique has been employed as useful means, since the emulsion does not incorporate water therein but contrarily, is slightly concentrated. In this technique, however, prospects for shortening a processing time is poor and a problem such as clogging of the filter has been further raised. As methods for solving these problems, is cited a process of applying ion-exchange, as disclosed in JP-A 61-219948, 62-23035, 63-40137, 63-40039 and 4-22942. Among these, a desalting process by applying electrodialysis is considered to be the most effective in desalting. However, a desalting method by applying conventional electrodialysis to the emulsion is insufficient in concentrating ability and therefore, an improvement thereof has been desired.
For the purpose of preparing uniform grains (e.g., in grain size, crystal habit, or halide distribution within the grain), there was proposed a fine-grain supplying method as disclosed in JP-A 1-183417 and 1-183645, and World Patents WO 89-06830 and WO 89-06831, in which there were not disclosed means for removing unnecessary salts from the fine grain emulsion. To desalt the fine grain emulsion, there was disclosed a process of applying the ultrafiltration in JP-A 2-172816 and 2-172817. These methods, however, have disadvantages as set forth in the above-mentioned ultrafiltration and an improvement is further desired.